The present invention concerns an endoprosthesis for a joint, particularly a finger, toe or hand joint, with a proximal and a distal joint part, of which one joint part in the flexion direction has an essentially convex contact surface around a body axis and the other has a correspondingly essentially concave contact surface, and which joint parts are joined by means of a flexible connection piece that takes up tensile forces, which piece has a thread-type or tape-type element.
The natural metacarpophalangeal joint (MCP) gives the finger a lateral freedom of movement, which differs each time depending on the flexion of the joint. In flexion, the degree of freedom is laterally zero to only a few degrees, but in extension it is approximately 30 degrees. In addition, a limited passive rotation is possible around the axis of the finger. The interphalangeal joint leaves free almost no movement play space laterally, independent of the flexion angle, and the joint parts can rotate only very slightly relative to one another. Therefore, each joint has its own particular degree of freedom with respect to extension and flexion, relative to rotation, but also relative to lateroflexion.
In order to permit natural movements of flexion, extension, and rotation around the joint, while, however, controlling the movement path of the finger opposite the metacarpus, in French patent application FR-A1-2,736,536, an endoprosthesis is proposed for a finger joint, which has a proximal joint part, a corresponding concave distal joint part and a connection and xe2x80x9cprogrammablexe2x80x9d part with a rod made of a pliant material. The rod sits in both joint parts in a guide channel, which is arranged axially in the pin of the joint part to be inserted in the bone marrow channel and begins with approximately cylindrical shape and spreads out into trumpet shape with decreasing distance to the condyle surface. The movement play space is defined by the interaction of guide channel and connection rod. Shape, dimensions and possibly the pre-programming of an alloy with xe2x80x9cshape memoryxe2x80x9d of the connection rod determine the course of movement of the joint.
Such a joint has the disadvantage that the pliant connection rod must fulfill a multiple number of tasks simultaneously. Thus it must be pliable differently in different directions, but it must be resistant to abrasion, and in any case, it must have a shape memory. It must have a certain stiffness, yet it must be flexible. In addition, the material should be compatible with the body and long-lasting, i.e., it should not be fatigued.
An orthopedic prosthesis implant is proposed in U.S. Pat. No. 5,534,033, in which the joint parts each have a cup that can be inserted into the bone marrow channel, and in the hollow cavity of this cup is attached a condyle of ceramics or carbon by means of an elastomeric adhesive, whereby the condyles contact one another in a sliding manner and both joint parts are joined by connection means. The connection means comprise either an envelope enclosing the condyles or a thread, which is guided through a hole in the condyles. Also, two threads at a distance relative to one another are proposed. These threads can be guided through holes in the condyles or also guided around the condyle. The function of the threads consists of guiding the joint parts in such a way that they are not dislocated. It cannot be derived from the document how the connection means react to a movement of the joint. It is assumed that the connection means must be either elastic (knitted DACRON fiber (trademark of E.I. duPont de Nemours and Company, Wilmington, Del.)) or are fastened to a spring. In any case, it can be derived from the drawings and the description that the distance between the attachment points of the connection means changes in length upon movement of the joint. However, this solution brings its own disadvantages. The threads, which are guided through the holes, can be sheared off by the edges of the holes at the condyle surface, or they can end up between the condyles and can be abraded therein. If the threads are guided around the condyles, they rub against the condyles and do not assure sufficient safety from dislocation when the joint is bent. The envelopes are stretched at any flexion on the upper side of the joint, or in the case of the extended joint, folded material of the envelope is present on the upper side.
It is thus the object of the invention to create an endoprosthesis for small joints, in which the mentioned disadvantages will be avoided, and a flexible connection of both joint parts that guarantees a play space will be assured. The connection will be loose enough and offers such little resistance, that the two parts can move freely, like the natural joint. The connection piece will not become fatigued. The movement play space of the joint will be definable with respect to rotation, extension, flexion and lateroflexion, and the latter will be dependent on extension, with a simultaneous loose holding together of the joint parts. An abrasion that is as small as possible will be assured.
According to the invention, this is achieved by the fact that the connection piece assures a defined play space between the contact surfaces, that a groove longitudinally extended in the flexion direction is formed in the convex joint part, that the connection piece is attached in this groove or at the base of this groove, whereby a lateral play space exists between groove and connection piece. In this way, there is a large freedom of movement in the flexion direction and simultaneously there is a definable, usually smaller freedom for a displacement or lateral deflection as well as a rotation. However, the joint is guided and protected against dislocation, since the connection of the two joint parts with a connection piece prevents a luxation.
When fibers of threads and fabrics or of flat tapes and thin membranes are flexed, the material is very slightly stretched and compressed in the direction of the flexion movement, due to the small dimension of the cross section. Fibers, tapes and membranes are consequently more suitable for flexible, pliant connection, the thinner the material cross-section. This property of fibers and tapes also holds true when they are spun and/or woven into large dressings or bandages, in order to be able to take up multiple tensile loads. Such bandages may be formed by spinning, weaving or knotting and have the shape of cords and ropes, tubings, tapes strips or flat woven fabrics. Essentially, the very small dimensions of the material in at least one dimension are suitable for flexion in one direction, and in two dimensions for flexion in several directions. The load capacity of such thread-type, tape-type or membrane-type parts makes them very useful for tensile designs. In addition, their resistance to fatigued fractures is assessed. High resistance to tearing with simultaneous resistance to fracture is realized and utilized, for example, in cords, ropes, nets, belts, tubings, membranes and foils of all types.
For endoprostheses for small joints, the use of such flexible thread-type, tape-type or membrane-type structures as the connection piece between the joint parts that move opposite one another is thus of great advantage. Such a connection piece makes possible a certain flexibility of the joint. Connecting threads or woven fabric parts can flex and even twist in all directions, without the danger thereby of an increased material wear or fatigue. A play space between the joint parts and therefore in the freedom of movement of the joint can be guaranteed without limitation. Also, an intense stressing of the joint by very frequent flexing and extending will hardly lead to material fatigue.
In joints, which should be held together by the connection piece over a lifetime, the thread-type or woven-fabric-type connection piece is comprised advantageously of a material that cannot be resorbed by the body.
Appropriately, there is a lateral play space between groove and connection piece, so that the connection piece is not sheared off at the joint part with a lateral movement, and does not scrape on the surface of the groove.
Appropriately, joint parts can be joined next to one another by several thread-type and/or woven-fabric-type connection pieces. In this way, a lateral deflexion of the joint is limited and the lever ratio between joint part and connection piece is favorably influenced.
Advantageously, an elevation is present in the concave joint part that works together with a depression e.g., a channel, furrow or groove in the convex joint part, and the lateral freedom of movement of the joint is limited by the assured play space between depression and elevation. In this way, abrasion-resistant materials are present at the contact points of the two condyles. Also, limitation of the freedom of movement can be designed by the shapes of the elevation and depression. The play space of movement relative to flexion in the principal flexion direction (flexion) and deflection crosswise to this (lateroflexion) can be very precisely defined by a depression in one condyle and an elevation, which cooperates with it, in the other condyle. This play space is also important for the permanence of the anchoring of the prosthesis in the bone. It reduces the transfer of shearing and transverse forces onto the anchoring of the prosthesis. These forces must thus, as a rule, be taken up by the tendons and the capsule tissue.
The surface of the elevation or the depression and the transition parts bounded thereby and the adjacent sliding surfaces or the actual joint surfaces are together characterized as the contact surface. The depression can coincide with the groove, but it can also be formed independent of it.
Advantageously, the lateral freedom of movement and/or the freedom of movement in the flexion direction of the joint is limited by the assured play space between sliding surfaces adjacent to the groove and sliding surfaces of the concave joint part interacting with the first surfaces. The freedom of movement can vary each time depending on the shape of the contact surfaces and depending on the length and attachment point of the connection piece, or according to the position of the point of rotation of the joint relative to the body axis of the convex joint part. Thus, the natural kinematics of the human joint will be reproduced. For the proximal (PIP) and distal (DIP) interphalangeal joints (IP joints) and toe joints, the sliding surfaces of the joint parts are advantageously at least partially cylindrical or truncated-cone-shaped, in order to prevent a lateral deflection of the members coupled by the joint. Preferably, there is such a sliding surface both for the convex condyle as well as for the concave condyle, comprised of two oppositely inclined truncated-cone surfaces with the same cone axis. In this way, a lateral displacement of the joint parts relative to one another can also be prevented.
Expressed in general terms, it can be stated that at least one part of the sliding surfaces with respect to their cross section, and appropriately on both sides of the connection piece, is formed in such a way that the places at which lines on one side of the connection piece that are perpendicular to the surface of these parts of the sliding surfaces approach or intersect the body axis, are set at a distance from the corresponding places of the corresponding lines on the other side of the connection piece. And in fact, these places may lie on the side of the connection piece, on which the corresponding sliding surface also lies, or, however, they may lie on the other side. Accordingly, the sliding surfaces run together toward the groove, such that they would form together a ridge or even a channel or a furrow in the absence of the groove. In this way, movement is limited by exercising pressure on the sliding surfaces and pulling on the connection piece.
For an angle of 90xc2x0 between the movement direction and contact surface, the limitation of movement is clear, and for small angles, the limit can be widened by stretching the connection piece. The limit of the freedom of movement is thus weaker, the smaller this angle.
Advantageously, an axis is present in the convex joint part, around which a loop of the connection piece is guided. In this way, the connection piece will not be flexed during flexion and extension of the joint, but it will slide, as a rule without load, around the axis, which has almost no effect on the aging of the connection piece, relative to wear and fatigue. A single axis can be present, or an axis can be present in both joint parts. The axes may be arranged rigidly or can be rotated in the joint part.
Advantageously, at least one of the joint parts is inserted by translation movement in a sheath attached in the bone. This sheath prevents the tensile forces, which occur very appropriately in the hand, from being transmitted to the anchoring of the joint part. Tensile forces must thus be taken up by the tendons and the tissue of the capsule. This has the advantage that the endoprosthesis cannot be torn from its anchoring. Such sheaths can also be designed such that a rotation of the prosthesis around the axis of the finger or toe will not be transmitted to the joint, but rather the joint part rotates in the sheath. The mounting of the sheaths is very simple as another advantage, and is conducted by screwing the sheath into the bone marrow channel.
Appropriately, the elevation is longer or shorter in the flexion direction, depending each time on the rotation play space to be guaranteed. A longer elevation permits a smaller rotation with the same play space.
Advantageously, the groove is broader or narrower, each time depending on the flexion angle, so that the lateral play space is different, depending on the position of the joint. In this way, possible lateroflexion can be controlled as a function of flexion.
The elevation advantageously surrounds the connection piece and thus the connection piece cannot rub against the contact surface of the condyle, particularly the depression.
Advantageously, the curve at the concave condyle, which curve is formed by the elevation and the adjacent sliding surface, in a cross section crosswise to the bending direction on one side of the joint, corresponds to the curve at the convex condyle, which is formed by the depression and the adjacent sliding surface. In this way, the contact surfaces lie at least linearly, and preferably flatly, against one another under load, so that high point loads and thus the increased abrasion that results are avoided.
Advantageously, the body axis of the convex joint part and the axis of rotation around which the concave joint can be pivoted, are distanced from one another. In this way, the play space between the sliding surfaces is different each time, depending on the flexion position. In this way, the lateral freedom of movement is also different, each time depending on the flexion position.
Advantageously, the sliding surfaces are spherical surfaces, the spherical centers of which are distanced relative to one another. In this way, a planar contacting of the sliding surfaces is practically given in any load case. For flexion without play space between the sliding surfaces, both spherical sliding surfaces of the concave condyle lie flat against the corresponding sliding surfaces of the convex condyle and move around the axis via the two spherical centers. In the case of lateroflexion and simultaneous flexion of the joint, one or the other sliding surfaces is flatly applied, depending each time on the direction of the lateral deflection, while the other sliding surface is removed.